Pituitary Hormones

The secretions of the pituitary gland are as follows:

• The anterior lobe synthesizes and secretes six principal hormones: follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), growth hormone (GH), and prolactin (PRL) (table 17.4). The first five of these are tropic, or trophic,9 hormones— pituitary hormones that stimulate endocrine cells elsewhere to release their own hormones. More specifically, the first two are called gonadotropins because their target organs are the gonads.

The hormonal relationship between the hypothalamus, pituitary, and a more remote endocrine gland is called an axis. There are three such axes: the hypothalamic-pituitary-gonadal axis involving GnRH, FSH, and LH, the hypothalamic-pituitary-thyroid axis

Hypophyseal Pouch Images

Telencephalon Diencephalon

Neurohypophyseal bud

Hypophyseal pouch

Primitive oral cavity

Embryo at 4 weeks

Sagittal section of (b) 4-week embryo

Embryo at 4 weeks

Sagittal section of (b) 4-week embryo

8 weeks

Fetus

Telencephalon Diencephalon

Neurohypophyseal bud

Hypophyseal pouch

Primitive oral cavity

Sella Turcica Embryology

Dura mater Sella turcica Posterior lobe Pars intermedia Anterior lobe -Sphenoid bone - Roof of pharynx

8 weeks

Fetus

Figure 17.3 Embryonic Development of the Pituitary Gland. (a) Plane of section seen in b. (b) Sagittal section of the embryo showing the early beginnings of the adenohypophysis and neurohypophysis. (c) Separation of the hypophyseal pouch from the pharynx at about 8 weeks. (d) Development nearly completed. The pars intermedia largely disappears by birth.

Dura mater Sella turcica Posterior lobe Pars intermedia Anterior lobe -Sphenoid bone - Roof of pharynx

Figure 17.3 Embryonic Development of the Pituitary Gland. (a) Plane of section seen in b. (b) Sagittal section of the embryo showing the early beginnings of the adenohypophysis and neurohypophysis. (c) Separation of the hypophyseal pouch from the pharynx at about 8 weeks. (d) Development nearly completed. The pars intermedia largely disappears by birth.

7 para = next to + ventricular = pertaining to the ventricle

Bsupra = above

Saladin: Anatomy & 17. The Endocrine System Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Nuclei of hypothalamus

Nuclei of hypothalamus

Hypophyseal Portal System

Axons to primary capillaries

Primary capillaries

Superior hypophyseal artery

Gonadotropin-releasing hormone Thyrotropin-releasing hormone Corticotropin-releasing hormone Prolactin-releasing hormone Prolactin-inhibiting hormone Growth hormone-releasing hormone Somatostatin

Secondary capillaries

Follicle-stimulating hormone Luteinizing hormone

Thyroid-stimulating hormone (thyrotropin) Adrenocorticotropic hormone Prolactin Growth hormone

Axons to primary capillaries

Primary capillaries

Superior hypophyseal artery

Secondary capillaries

Superior Hypophyseal Artery Images

Cell body

Portal venules

Figure 17.4 Anatomy of the Pituitary Gland. (a) Major structures of the pituitary and hormones of the neurohypophysis. Note that these hormones are produced by two nuclei in the hypothalamus and later released from the posterior lobe of the pituitary. (b) The hypophyseal portal system. The hormones in the violet box are secreted by the hypothalamus and travel in the portal system to the anterior pituitary. The hormones in the red box are secreted by the anterior pituitary under the control of the hypothalamic releasers and inhibitors. Which lobe of the pituitary is essentially composed of brain tissue?

Cell body

Portal venules

Posterior pituitary

Anterior pituitary

Figure 17.4 Anatomy of the Pituitary Gland. (a) Major structures of the pituitary and hormones of the neurohypophysis. Note that these hormones are produced by two nuclei in the hypothalamus and later released from the posterior lobe of the pituitary. (b) The hypophyseal portal system. The hormones in the violet box are secreted by the hypothalamus and travel in the portal system to the anterior pituitary. The hormones in the red box are secreted by the anterior pituitary under the control of the hypothalamic releasers and inhibitors. Which lobe of the pituitary is essentially composed of brain tissue?

Saladin: Anatomy & I 17. The Endocrine System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

642 Part Three Integration and Control

Table 17.3 Hypothalamic Releasing and Inhibiting Hormones that Regulate the Anterior Pituitary

Hormone

Principal Effects

TRH: Thyrotropin-releasing hormone

Promotes TSH and PRL secretion

CRH: Corticotropin-releasing hormone

Promotes ACTH secretion

GnRH: Gonadotropin-releasing hormone

Promotes FSH and LH secretion

PRH: Prolactin-releasing hormone

Promotes PRL secretion

PIH: Prolactin-inhibiting hormone

Inhibits PRL secretion

GHRH: Growth hormone-releasing hormone

Promotes GH secretion

Somatostatin

Inhibits GH and TSH secretion

involving TRH and TSH, and the hypothalamic-pituitary-adrenal axis involving CRH and ACTH (fig. 17.5).

• The pars intermedia is absent from the adult human pituitary, but is present in other animals and the human fetus. In other species, it secretes melanocyte-stimulating hormone (MSH), which influences pigmentation of the skin, hair, or feathers. Humans, however, apparently produce no circulating MSH. Some anterior pituitary cells derived from the pars intermedia produce a large polypeptide called pro-opiomelanocortin (POMC). POMC is not secreted but is processed within the pituitary to yield smaller fragments such as ACTH and endorphins.

• The posterior lobe produces no hormones of its own but only stores and releases OT and ADH. Since they are released into the blood by the posterior pituitary, however, these are treated as pituitary hormones for convenience.

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Responses

  • efrem asfaha
    Which lobe of the pituitary is essentially composed of brain tissue?
    6 years ago

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